1. Field of the Invention
This invention relates generally to an instrument for measuring light reflectance from a surface, and more particularly to such an instrument for measuring light reflectance from fabric and the like materials which have a heterogeneous surface configuration.
2. Prior Art
A number of instruments are presently available for measuring light reflectance from a surface. Generally, such instruments employ a single light source and a single light sensor mounted with respect to one another such that the sensor will receive only diffused light from the surface under investigation. Usually this is accomplished by mounting a light sensor on a line which is normal to the surface under investigation and a light source on a line which is at 45.degree. with respect to the normal. Of course, the light source and the light sensor can be interchanged in their positions to obtain the same effect.
However, this arrangement is not satisfactory for investigating the light reflectance of materials having a heterogeneous surface configuration, such as fabrics. The problems encountered with fabrics are caused by the heterogeneous surface configuration thereof consisting of overlapping and intertwined threads or fibers. Modern fabrics, such as double knit fabrics, present the most serious problems, due primarily to the relatively large gradient of porosity which exists at different angular orientations thereof and to the relatively glossy fiber surfaces which produce nonrandom specular light at angles other then expected angles of specular reflection from the surface of the material.
Fabrics consist of a plurality of thread extending in a longitudinal direction, commonly called the warp, and a plurality of laterally extending threads, commonly called the woof or weft. Some fabrics, however, have a more complex structure then a simple warp and weft, such as double knit fabrics. These fabrics exhibit a venetian blind effect in which the light transmission through the fabric and, therefore, the light reflectance of the fabric varies in accordance with the angular displacement of a light source from a line which is normal to the fabric surface. It has been found, for example, that many fabrics exhibit a relatively high light transmission when the angle of impinging light rays is 45.degree. with respect to a normal.
Because of the warp and weft of a fabric, the light transmission characteristics thereof vary in accordance with the angular position of such a light source with respect to the warp and weft. That is, if one were to observe the light transmission through a fabric while the fabric is rotated, a gradient of light intensities would be observed which would alternately increase and decrease.
Accordingly, if one of the above mentioned light reflectance instruments is employed to measure the light reflectance from a material having such effects, different readings would be obtained for different angular orientations of the instrument with respect to the material. It can be readily appreciated that such a variation cannot be tolerated. These problems are also enhanced by nonrandom specular light reflected from the relatively glossy surfaces of synthetic threads or fibers. It has been found that light reflectance from fabrics can vary as much as 5% because of the above mentioned effects.
The variation in random specular light in accordance with the orientation of the light source and sensing unit to the material under investigation can be reduced by decreasing the size of the sensing area. However, because of the different types of individual components forming the heterogeneous surface of a material, a relatively small sensing area in the order of several fiber diameters will not produce uniform results at various locations on the same material. Accordingly, a relatively large sensing area must be employed such that the diffused and specular light from all of the different types of individual components forming the heterogeneous surface will be integrated.
It has also been found that doubling the number of light sources will not overcome the above mentioned problems, since diametrically opposed illuminating or sensing positions with respect to the sensing area will each have the same results. Accordingly, it can be appreciated that a need exists for an instrument which is not sensitive to the above mentioned effects. Because of the relatively large sensing area required for materials having a heterogeneous surface configuration, these effects cannot be completely eliminated without complex and expensive optical techniques. However, it has been found that these effects can be reduced with relatively uncomplicated and inexpensive optical techniques to such an extent that they are negligible and with reasonable limits for all practical purposes.